1. Summary of the Invention
This invention relates to a method and apparatus for selectively screening particulate matter from produced fluids. More particularly, a well screen is arranged to pass particulate matter of a predetermined size which will not substantially erode or block equipment in the flow path of the produced fluids. Further, the invention relates to a well screen which will increase the production of fluids from a well.
Commonly, a drilling mud is used when drilling a well into an earth formation for the production of fluids. When drilling a hydrocarbon well the drilling mud acts as both a lubricant and a borehole stabilizer. Typical drilling muds are comprised of water, polymers, starch, barite, bentonite and additional additives which may be chosen depending upon the environment of the well.
While the drilling mud serves a useful purpose in the drilling of a borehole, the presence of the mud during the production of fluids may be detrimental. Typical drilling muds will form a filter cake on the walls of the borehole. This cake consists of small particles some of which were originally a part of the mud. Depending on the drilling mud used and the characteristics of the formation, these particles are usually 100 microns or smaller. Due to the high pressures incurred during drilling this cake is very dense and compact and is capable of precluding the flow of fluids. Therefore, it is desirable to remove the cake prior to producing fluids from the well. However, due to the nature of the cake it is not easily removed and will commonly plug filters placed in the well production system. As a result several workover trips may be required to replace or clean the filters. These procedures delay production of fluids and increase costs as the expense of replacing a filter in an offshore oil well may be several million dollars not including the revenue lost by the delay in production.
2. Background of the Invention
Prior to the present invention, the preferred method of producing fluid from wells has been to remove all or substantially all solid particles from the fluid before it entered the production pipe. Over the years several different methods and filters have been introduced. Some of these methods include gravel packing the well, use of prepacked filters which may utilize sand, gravel or another media, or use of fibrous materials. In each instance, the primary goal has been to preclude passage of solid particles through the production pipe string. The following provides a brief summary of apparatus used to achieve this goal.
U.S. Pat. No. 3,768,557 describes a graded multi-layer pre-packed sand filter for oil and other fluids containing sand. At col. 1, line 15, the stated purpose of the filter is to remove the sand from the liquid before it is produced from the well bore.
U.S. Pat. No. 4,34,054 describes a filter which is capable of segregating out the solid component of a fluid flow. The filter comprises a bed of randomly disposed fibrous members which retain solid particles and allow the particle free fluid to pass.
U.S. Pat. No. 4,917,183 describes a pre-packed gravel packing screen. The screen has a filtering bed comprising a fluid permeable bed of particulate solids. The particulate solids are sized to effectively prevent all the particulate matter in the well production fluids from passing inwardly through the bed into the well conduit.
While in the past it was believed that all solid particles must be excluded from the produced fluids, the present invention provides a method and apparatus which allows for the economical production of fluids containing particulate matter. Due to the high cost of replacing a well filter, production of fluids containing particles of sand and other material is desired if the procedure is not detrimental to the production system.
One reason for filtering solids from the produced fluids is that ultimately such solids must be removed from the product that is passed to a sales line. This is usually accomplished by various separation techniques such as gravity vessels, centrifuges, hydrocyclones, etc. While these processes are costly and the equipment requires valuable space, especially on an offshore platform, these expenses are quickly exceeded by the costs of the workover procedures which must be performed to replace or remediate plugged filters.
Thus, one object of the present invention is to provide a filter assembly which selectively allows for the initial production of fluids containing particulate matter. The particles to be produced have been predetermined to only minimally damage the interior of the production system. Further, following this initial period, the filter of the present invention will specifically allow the production of those particles which will not damage, obstruct or erode the interior of the production system while precluding the production of potentially damaging particles. The present invention also includes a method for filtering particles from produced fluids. According to this method, various apparatus may accomplish the filtering scheme of this invention, i.e. to pass those solids which will not damage equipment during production of fluids from the well.